1. Field of the Invention
The present invention relates to a lead-acid battery, and in particular to a lead-acid battery that uses plates each including a current collector which is unitarily provided with one or more current collecting lug parts and holds an active material thereon and to a method for manufacturing a current collector for a lead-acid battery.
2. Description of the Related Art
A conventional lead-acid battery is manufactured as follows. A positive plate and a negative plate are prepared. The positive and negative plates each include a current collecting portion unitarily formed with one or more current collecting lug parts at the upper edge thereof and holding an active material thereon. A separator is disposed between the positive plate and the negative plate to form a plate group. Then, the plate group is received in a battery case and an electrolyte is injected into the battery case to obtain a desired lead-acid battery.
There are two types of plate groups, namely, a layered structure type and a wound structure type. The plate group having the layered structure is formed by alternately layering at least one positive plate and at least one negative plate one by one with a microporous separator interposed between adjacent plates. If a plurality of plates of the same polarity are used to form a positive plate group and a negative plate group and the positive plate group and the negative plate group are combined to form a plate group. Current collecting lug parts of positive plates included in the positive plate group are provided on one side across the center line in a layering direction in which the positive plate, the negative plate, and the separator are layered. Current collecting lug parts of negative plates included in the negative plate group are provided on the other side across the center line in the layering direction. Thus, a strap is welded to the current collecting lug parts of the plates having the same polarity to connect the plates having the same polarity to each other.
The plate group having the wound structure is formed by winding one elongated positive plate and one elongated negative plate with a microporous separator interposed between the plates. If a plurality of current collecting lug parts are unitarily provided at a plurality of locations of the elongated positive plate and the elongated negative plate, a strap is welded to the plurality of current collecting lug parts of the plates having the same polarity to connect the plates having the same polarity to each other.
In the lead-acid battery, the positive current collector may be subjected to oxidative corrosion while in use so that the positive plate may be expanded in a height direction (the direction from the current collecting portion toward the current collecting lug part). Since both ends of the plate group in the height direction are restricted in the battery case, a stress due to the expansion may concentrate on the current collecting lug part which has a relatively low strength. As a result, stress corrosion may occur at the current collecting lug part where a stress concentration occurs, thereby accelerating corrosion. In a lead-acid battery mounted on a mobile object such as an automobile, the current collecting lug part is always subjected to a stress due to vibration while in use.
In view of the foregoing, it is feared that the current collecting lug part may be inconveniently broken. In order to eliminate such a risk, the following techniques have been proposed. Japanese Patent Application Publication No. 2003-297414 discloses that a portion is provided at the lower edge of a current collector to be deformed by a stress produced when a positive plate is expanded in the height direction and then to absorb the stress. Japanese Patent Application Publication No. 11-329373 discloses that a thin-walled portion is provided in a battery case so as to be extended following an extension of the positive plate when the positive plate that expanded in the height direction. Japanese Patent Application Publication No. 2008-171701 discloses that the entire surface of a current collecting lug part of a negative current collector is covered with an insulating cover member to prevent a breakage due to vibrations and further to prevent a short circuit even if a breakage should occur.
Further, Japanese Patent Application Publication No. 07-326359 discloses that a rib is provided to extend from the boundary between a current collecting lug part and a current collector frame in the longitudinal direction of the current collecting lug part. The rib is provided for the purpose of preventing the current collecting lug part from being bent before a step for manufacturing a plate group of a lead-acid battery (a step for forming a strap on the current collecting lug parts by burner welding) is performed. The current collecting lug part provided with the rib is configured to be able to withstand a bending pressure to which the current collecting lug part is subjected before the step for manufacturing the plate group is performed. According to the invention disclosed in Japanese Patent Application Publication No. 07-326359, the current collecting lug part is not bent before the step for manufacturing the plate group is performed. Thus, when the current collecting lug parts are set with respect to a metal pad and a metal comb (in a mold for forming a strap) in the step for forming a strap on the current collecting lug parts by burner welding, the current collecting lug parts are not cut, twisted, or otherwise made defective by the slit surface of the metal comb when the current collecting lug parts are bent. The width of each slit of the metal comb is equivalent to the thickness of each current collecting lug part to be inserted into the slit. Therefore, as a matter of course, with the current collecting lug part set with respect to the metal comb, the upper end of the rib extending along the longitudinal direction of the current collecting lug part is located below the lower surface of the metal comb. That is, in a portion of the current collecting lug part that is located directly below the strap formed on the current collecting lug part, the current collecting lug part is not formed with the rib for at least a length that is equivalent to the thickness of the metal comb.